Uranium mineralization in oxidized fractured environment of the giant volcanic related uranium field from the Krasnokamensk area

Modification of primary to secondary uranium mineralization in oxidizing volcanic environment is a phenomenon, which is necessary to be understood in terms of survey and exploration of mineral resources, remediation of territories contaminated by radionuclides, and as natural analogues in the context of safety assessment of the nuclear waste facilities. The Mesozoic (135 Ma) welded tuff unit of volcanic strata, which have hosted the large Tulukuevskoe U deposit (∼35k tons U) in the Krasnokamensk area, Russia, provide striking examples of processes governing U migration and accumulation in oxidizing conditions. The vein-type deposit contains primary mineralization occurring as pitchblende subjected to secondary transformations within the upper part of the deposit, mined in the Tulukuevsky open pit (TOP) during 1972-1998. There are two types of ores such as pitchblende (U) confined to steeply dipping Fault 1A and pitchblende-molybdenite (U-Mo) developed along flat fault located at the bottom of the TOP. Reconnaissance investigations at seven levels of the TOP have shown that the pit NW wall is the most promising block (∼ 200 x 200 x 200 m) to observe the transformations of the primary U mineralization in unsaturated conditions (vadose zone) of the tuffs. The main aim of this contribution is to define the primary controls of the hydrothermal mineralization, the preferential pathways for meteoric water infiltration, mineral-chemical transformation of country rocks, and alteration of U mineralization in the context of redox front propagation through unsaturated fractured porous tuffs with application of the data to modeling of U migration and deposition of secondary concentrations using quasi-stationary state approximation (QSSA) approach. It is concluded that fractured porous environment such as welded tuffs requires specific conceptual and numerical treatments because both the fractures and porous matrix are active parts of the flow and transport regime as well as development of secondary U mineralization. In the context of detection of spatiotemporal relations between redox front propagation and geochemical events, which have occurred in the vadose zone of the TOP, the QSSA approach is the most convenient. This approach is based on the assumption that meteoric waters penetrating the rocks react with the latter during the period required for the equilibrium into the waterrock system with development of the adequate rock forming and U mineral association. It correlates with the stationary state in the elementary volume through which meteoric waters infiltrate gradually. All the blocks are characterized by equivalent fracture network geometry and specific mineral-chemical composition organized with depth. Obtained data on U repartition as function of initial metasomatic and secondary oxidizing transformations, mineral surface areas, and meteoric/fracture water chemistry with accessible thermodynamic parameters could be used as initial/eventual conditions. It is evident that QSSA approach will allow more reliable and detailed description of the dynamics of the redox front propagation and thus the identification of the conditions of U transfer in oxidizing fractured unit as well as deposition of secondary U concentrations at the TOP. These data could be applied for developing thermodynamic geochemical and numerical reactive transport models for deeper insight into the processes of actinide migration in the natural environment